Run GCC and Clang on Travis-CI.

I tried to use the so-called options files, but those weren't working. I don't really like this build system very much because it won't do what it advertises and is incredibly inflexible. I'm not advocating changing this late in the game, I'm just complaining.

Basically, I copied the existing `project.yml` file to `gcc.yml`. I then use an environment variable to make Ceedling use the `project.yml` or `gcc.yml`. The default (`project.yml`) is still to use Clang.

If you make any changes in `project.yml`, make sure to mirror them in `gcc.yml` as appropriate.